Pile Hammer Size / Driving Criteria

Pile Hammer Size / Driving Criteria

Pile Hammer Size / Driving Criteria

I was taught to size a pile driving hammer so that the pile reaches the desired capacity at approximately 10 to 12 blows per inch. (I know that some agencies require all piles to be driven to refusal at 20 blows per inch). Lately, it seems that people are driving piles to capacity with larger pile hammers and much fewer blows per inch, such as 1 to 4 blows per inch.

On a recent project I was involved with, GRL ran a WEAP analysis that told the contractor to drive the 75' long , 100 kip ultimate capacity, HP12x53 piles 70 feet through soft organic silts, through medium sand, into denser sand at only 10.2 blows per foot with an APE D19-42 diesel hammer having 41.8 ft-kips maximum rated energy. I reviewed the WEAP printout and everything looked OK. However, it seems to me that 10 blows per foot is not enough resistance to confidently determine the pile's ultimate capacity. I personally would have called for a smaller pile hammer or turned down the fuel setting in order to raise the required blows per inch. I like to see a pile "take up" as the blow count increases.

What amount of hammer blows are you looking at when sizing a hammer and determining ultimate pile capacity? Comments?

RE: Pile Hammer Size / Driving Criteria

I am not very experienced with pile foundation. However, I am currently involved with installation of piles using the same type and size of hammer, and the same type of piles (HP 12x53). The ultimate capacity of the piles are suppose to be 225kips. WEAP analysis and PDA analysis were conducted. The recommendation was drive to 25bpf, which was within the range of the WEAP analysis. Based on your ultimate capacity of 100kips, 10bpf seems reasonable.

RE: Pile Hammer Size / Driving Criteria

Thanks for your reply, Riggly. It seems to me that 10 bpf are too few to know how well the pile capacity is developing - so are your 25 bpf. The hammer is big. The pile is moving through the soil relatively quickly, too quickly, I feel. You have almost the same situation that I have. The difference is that you may not have enough experience to question the significance of pile blow count and rate of pile advancement. Please don't take this as a knock against your knowledge or experience. I'm hoping to get some comments from others who do a lot of pile design and construction.

Your pile capacity is much higher than mine and that's why you have an HP12x53 pile. My client is using an HP12x53 pile only because he has them left over from another building on the same project where the pile capacity was greater.

RE: Pile Hammer Size / Driving Criteria

I would also like to see more than 10 bpf as a cutoff critteria. I would also expect the contractor to have trouble driving the piles through the softer soils. The pile will likely try to "run" when struck. This can create a slow and difficult situation for th contractor just to get the pile to the relatively dense sand.

RE: Pile Hammer Size / Driving Criteria

PEinc – This is a very interesting question. The US Army Corps of Engineers, Engineering Instructions for Pile Driving Equipment implies that the acceptable blow count range, as determined by GPLWEAP, is from 5 blows / foot to 240 blows / foot. I downloaded this document in .pdf format some time ago, but it no longer seems to be available. If you don’t have it, will be happy to email it.

Since pile driving is as much “art” as it is “science”, I’ll give you my “opinions” from the “art” viewpoint of former bridge construction contractor who drove piling in soft soils (South Carolina coast) with a similar (but smaller & older) single-acting diesel hammer (McKiernan-Terry DE-30).

For starters, the 4190 pound ram weight of APE D19-42 compares favorably with (exceeds) the pile weight of 3980 pounds.

I agree with you that 10 bpi, or so, is typically taught as the “target” for good practice. This seems to be based on over a century of success with the Engineering News formula using single-acting air/steam hammers such as a Vulcan #1 (5000 lb. weight falling 3 feet – 15,000 ft-lb, no matter what the soil conditions). A single-acting diesel has very different characteristics. Even though the APE hammer has a maximum rated energy of about 41,800 ft-lb, the energy delivered can be substantially less in soft soils (even at the same fuel setting). I downloaded the APE hammer specs & performance data – it has four fuel settings. In very soft soils the rating for a diesel hammer is essentially zero ft-lb – the pile / hammer moves too much to allow the falling ram to compress the fuel-air mixture for combustion.

At 10 bpf, I would guess (and it is only a guess) that this hammer is delivering about 25,000 to 30,000 ft-lb to the pile (6 or 7 ft stroke). With this type diesel hammer, maximum energy is seen only at practical refusal.

With 70 feet of the pile in soft organic silts & medium sands, I would expect skin friction to be a major factor for both driving resistance and final capacity. A fairly large hammer is the needed to overcome the friction during driving – the “penalty” is that there is significant movement with each blow. However this “penalty” is an illusion – when driving is stopped, skin friction will rapidly reestablish itself to provide bearing capacity (I’m certain you know why & how better than I do).

The reason to stop driving at what seems to be a fairly high penetration rate (10.2 bpf) almost certainly has to do with the denser sands at the pile tip. You don’t want to risk going through a good layer like this. Point bearing capacity is probably minor in this case anyway. In my experience most pile failure come from overdriving, not underdriving.

In summary, the APE D19-42 hammer sounds like a good choice for this job, you say that WEAP printouts look good, and the contractor sounds like he is “following instructions” – I’m willing to bet that you have good piles that meet the design criteria. Of course a load test, if possible, will tell.

Best Wishes

RE: Pile Hammer Size / Driving Criteria

I don't think I have the pdf you refer to but i do have the ASCE/USACOE Design of Pile Foundations manual. Table 5-2 is titled Limiting Penetration Rates. The table gives recommended maximum blows per inch for various pile types. For steel H-Piles and Pipe Piles, the maximum rate is 10 - 20 bpi. I assume your pdf has the same or very similar information. If not too much trouble, you could e-mail the pdf to me at your convenience. Send it to PeirceEngr@aol.com. Thank you.

The GRL WEAP analysis assumed 50% skin friction. CAPWAP analysis and dynamic testing were done by GRL on similar some piles for the immediately adjacent building. There was no concern about punching through the dense sand. The sand gets denser and then turns into decomposed rock and then intact rock. At around 10 bpf, the D19-42's stroke is about 5.5' with less than 19 kip-ft of energy.

I can check with the contractor, but it seems to me the contractor already had the D19-42 hammer and told GRL to use it. I'm sure the contractor did not think of reducing the fuel setting, after all, the contractor wants to drive the piles as fast as possible. I'm just surprised that GRL did not recommend a lower fuel setting.

The piles I am involved with are to be driven through about 10' of granular fill, then through 35' or 40' of the soft organic silts and silty clays, and then into dense sands and gravel. In the organic silts and silty clays, N = 3 to 4 bpf. Beneath this was sand with N > 40 bpf. According to my FHWA Driven analysis, the majority of capacity is from end bearing, not friction.

The project engineer is happy with the WEAP analysis and driving criteria. If I had done the WEAP, I would have tried using a lower fuel setting and or a smaller hammer to see if I could increase the blows. I'd like to see the hammer work a little bit harder.

RE: Pile Hammer Size / Driving Criteria

Turning down the fuel supply would be interesting, I don't think that it would make any difference at the driving conditions that you describe. Reduced fuel should only limit how "hard" the hammer hits at practical refusal. Even with reduced fuel, this hammer can probably deliver 19,000 ft-lb (5.5 ft stroke) that the pile is "calling for", and it will do so.

The self-regulating characteristics of a diesel hammer are remarkable. Our DE-30 did not have an adjustable fuel supply. In soft driving conditions the fuel would ignite before being reaching optimum compression. The result was poor combustion, a lot of smoke, and unburned diesel fuel being "spit" all over the place. In other words, if the fuel is not needed for coversion into kinetic energy, the hammer finds a way to "ignore" it. If more energy is needed (hard driving conditions) the hammer improves it's own efficency, and is then limited by the amount of fuel supplied.

RE: Pile Hammer Size / Driving Criteria

RE: Pile Hammer Size / Driving Criteria

I wasn't really concerned with determining pile capacity. My original question was: What amount of hammer blows are you looking at when sizing a hammer and determining ultimate pile capacity?

I want to know how comfortable people are with big pile hammers and low blow counts per inch. At what minimum number of blows per inch should the pile hammer drive the pile to obtain the desired ultimate capacity? I don't like seeing a really powerful pile hammer banging a pile through the ground with minimal resistance.

RE: Pile Hammer Size / Driving Criteria

The following was excerpted from Estimating Driven Pile Capacities During Construction, Chapter 2, Pages 5 & 6, by the University of Florida's McVay, Alvarez, Zhang, Perez, and Gibson, August 2002, submitted to Florida DOT.

Blow count criteria.Using the Wave Equation Analysis for Piles (WEAP) the engineer can determine the number of blows per specific penetration to reach a design pile capacity. The blow count has to be averaged for every 12 inches (250 mm) of pile penetration or through the last 10 to 20 blows of the hammer. It should be noted that the driving equipment must be selected in order to provide the required resistance at a blow count ranging from 36 blows per foot (36 blows for 300 mm) to 120 blows per foot (120 blows per 300 mm).

Practical refusal.Practical refusal is defined as a blow count of 20 blows per inch (20 blows per 25 mm) for 2 inches (50 mm) of driving. The FDOT specifications recommend that driving cease after driving to practical refusal conditions for 12 inches (300 mm). If the required penetration can not be achieved by driving without exceeding practical refusal, other alternatives should be considered such as jetting or Preformed Pile Holes.

RE: Pile Hammer Size / Driving Criteria

An interesting thread.I think a lot of it depends on what you are doing with the pile, the capacity of the pile and the soil conditions. In this case the pile is not in danger of refusing, so turning down the fuel setting will only decrease the set, slowing production and somewhat reducing pile capacity.At 100 kips ultimate capacity and an FS of 2.5, the allowable load should be 20 tons, which is not a large pile. Once you found the pile in dense sand 70 feet deep, you should be able to develop the 20 ton capacity easily. I would do a static analysis to check the pile capacity. The hammer was probably selected because it has a good sized ram so it can follow the pile through the organics and soft soils. The contractor was proably concerned the smaller hammer might stall out in the soft overburden.I do agree that if the pile capacity were higher and for say a building foundation that I would want a hammer that would give me 4-10 blows to the inch at capacity.

RE: Pile Hammer Size / Driving Criteria

When evaluating a pile hammer NYDOT usually likes to see the blow counts between 20 BPF and 120 BPF for rated resistance (Skin Friction) pile. Also, the capacity blow count is recommended to be sustained for 1.5 feet. For a practical refusal (end bearing) pile, we recommend a blow count of 20 BPI.

I had a job where I wanted to reduce the stroke on a hydraulic hammer to increase the the blow count. We were unable to reduce the stroke because the piles were to be driven during a overnight track outage and there were concerned about getting all the pile driven in time.

If you are concerned, I recommend you contact the engineer that submitted the pile driving critiera. There maybe a reason why he specfied that blow count.

RE: Pile Hammer Size / Driving Criteria

RE: Pile Hammer Size / Driving Criteria

First, 10 to 12 blows per inch is really outlandish even though the criteria has been around quite some time. One is driving a stake with a tack hammer at that condition. The hammer is being damaged, the pilebuck foreman rightly thinks you are crazy, and the inefficiency is high. Forget it.

I personally like 12 to 24 blows per foot as it is fast enough to prevent boredom while watching, yet slow enough to enable me to separate set and rebound per blow. In your case the lump weight (striking parts) is well matched to the pile and conditions and results should be favorable. Do not short-stroke the hammer. Get up on it. Hit the pile.

From your description I expect the pile may run thru the silts, begin to improve in the medium sands and do nicely in the dense sands. 10 blows per foot remember is the minimum and you go up from there. And you have good room to go up. Embed the stick a few feet in the dense sand at blow count and call it a pile.

I suspect a PDA restrike test will show you much more than a 100kip service load for these piles installed in this way.